This invention is generally in the field of optical communication techniques, and relates to an integrated optical device particularly enabling frequency selection, switching, modulation and signal routing.
Optical communication is the enabling technology for the information age, and the essential backbone for long haul communication. As this technology progresses, there is a tremendous interest in providing optical routes in the short haul, metropolitan and access networks, as well as in local area networks and cable TV networks. In all these networks, the best of breed solution for bandwidth expansion has been the adoption of wavelength division multiplexing (WDM), which entails the aggregation of many different information carrying light streams on the same optical fiber. A device capable of accessing an individual information stream is fundamentally required in current and future networks. These devices can also add information streams to the optical fiber, as well as impress information on an optical stream by optical modulation.
The basic building block for optical switching is the optical modulator/switch. Various implementations of such a device have been developed, of which the most dominant is the Mach-Zender Interferometer (MZI), in which interference is produced between phase coherent light waves that have traveled over different path lengths. The basic construction of MZI is schematically illustrated in FIG. 1. Light input to a modulator 1 is via a single-mode waveguide. A beam splitter divides the light into two equal beams that travel through guides 2A and 2B, respectively. By applying voltage to electrodes 4, the effective path lengths can be varied. Hence, the optical switching is achieved by creating a phase difference between two arms of the device (guides 2A and 2B), and controlling the optical power at the device output.
In general, the performance criteria for the operation of the wavelength routing elements include: the following:
(1) Modulation depth or contrast ratio, which signifies the ratio between two (xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d) or more states of a switch device;
(2) Crosstalk, which defines the ability of the device to select a single optical channel while suppressing information from the other channels;
(3) Electric power consumption;
(4) Modulation bandwidth, which defines the speed at which the switching can be achieved; and
(5) Optical bandwidth within which the modulation is effective.
To achieve a good modulation performance with the MZI, the latter is typically designed with long interference arms. As a result, this device is not efficient in its implementation, and limits the scaling ability of complex optical circuits. Another drawback of MZI-type devices, in their predominant implementation, is their frequency insensitiveness over a desired frequency bandwidth. As a result, MZI-type devices cannot be used directly for wavelength routing.
To achieve wavelength routing, the MZI has been utilized in conjunction with wavelength demultiplexers, which provide spatial separation between different optical frequencies. To this end, a matrix composed of at least N times (N+1) MZI is used to redirect one of the N spatially distinct wavelengths to the device output. The remaining frequencies are recombined using a wavelength multiplexer.
Recently developed integrated electro-optical devices utilize resonant rings to achieve frequency selective switching. Such a device is disclosed, for example, in WO 99/17151. The main components of the device are illustrated in FIG. 2. A resonant ring 6 couples light from one fiber 8a to another fiber 8b, when the frequency of the light passing through the fiber 8a fulfils that of the resonance condition of the ring 6. By applying an electric field or a thermal source to the ring 6, its refractive index, and consequently, its resonance condition, can be desirably adjusted. Changing the resonance condition prevents the passage of the previously coupled light and acts as a switch. Alternatively, the loss of the ring waveguide can be changed. Adding loss to the ring diminishes its operation as a resonant cavity, and light cannot be coupled from fiber to fiber.
Unfortunately, the conventional resonant ring based systems require fabrication tolerances that are hard to implement by means of a conventional photolithography technique. This disadvantage becomes more essential in multiple-ring devices, wherein the distance between two locally adjacent rings is a critical factor for the successful operation of the device. The use of a switching mechanism providing de-tuning of a resonant ring out of resonance condition has been proposed, being disclosed for example in WO 98/53535. This solution, however, does not meet the extinction ratio and crosstalk requirements of communication systems.
There is accordingly a need in the art to improve the operation of electro-optical communication devices by providing a novel electro-optical device such as an optical frequency dependent switch and a modulator.
The present invention takes advantage of the use of several (at least two) ring resonators. The main idea of the present invention is based on designing an optical complex filter/resonator, wherein waveguide sections are specifically connected to ring resonators in a configuration which enables realization of optical switching, wavelength routing, lasing, wavelength sensitive amplification and optical filtering. The device may also combine a plurality of such filters in a wavelength router module. Generally speaking, the present invention utilizes the collective response of two or more closed loop resonators, which are connected to each other by two or more optical paths, for the purpose of switching or modulating a selected wavelength.
The fabrication technology of waveguides is well developed in many classes and families of materials. The relaxation of the fabrication tolerances in the present invention relates to the possibility of vertical coupling of light from the waveguides to the ring resonators. Since the vertical fabrication tolerances are much better then the horizontal tolerances, the result is a device which is simpler to manufacture. However the details and design of the invention extend beyond such devices in which only vertical coupling exists between the waveguides and resonators.
The optical resonator according to the invention is an enclosed cavity aimed at storing optical energy. As compared to the known devices of the kind specified, which utilize a closed-loop type optical resonator, and several such resonators cascaded in various configurations, the present invention utilizes the inclusion of a feedback path for the optical signal. In other words, in the present invention, the loop resonator serves as a frequency selective mirror within a more complex resonator. These mirrors together with connecting waveguides create closed loop cavities with superior performance and simpler fabrication, and, mainly, are favorable for inclusion of switching capabilities and active media, as compared to the conventional devices.
The wavelength response of a structure composed of several ring resonators coupled to optical waveguides is determined by the physical and geometrical parameters of the resonators and coupling scheme. The present invention provides novel schemes of coupling multiple resonators to achieve predetermined active filtering and modulation characteristics. These coupling schemes are relatively easy to implement, and provide desired modulation characteristics.
There is thus provided, according to one aspect of the present invention, an optical device comprising:
(a) a combination of two spaced-apart waveguides and at least two spaced-apart resonator-cavity loops accommodated between the two waveguides and connected to each other through sections of the waveguides, said at least two spaced-apart resonator-cavity loops and said waveguide sections creating a closed loop compound resonator for storing optical energy of a predetermined frequency range; and
(b) a control means for controlling physical characteristics of the compound resonator to adjust its optical storage characteristics.
The predetermined frequency range is determined by physical and geometrical characteristics of the compound resonator. To control the physical characteristics of the waveguide and/or loop-resonators, a heating means may be used.
One of the two waveguides serves as an input and throughput waveguide, and the other serves as an output waveguide. An optical signal entering the input waveguide may include a plurality of light components having different wavelengths. By actively adjusting the response of the compound resonator, using a heater or any other means that changes the characteristics of the waveguide sections, one of these wavelengths may be switched from the input to the output waveguide.
The device may comprise additional waveguides and additional loop-resonators, forming together several such frequency selective switches, thereby providing complex optical signal switching and routing.
According to another aspect of the present invention, there is provided a wavelength router system comprising at least one optical switch and at least one optical filter, wherein the switch and the filter is constructed as the above-described integrated electro-optical device.
According to yet another aspect of the present invention, there is provided a laser device where an active material with gain is embedded in at least one of the parts comprising the above-described integrated electro-optical device. This multiple section laser can be controlled by applying the above-described control means to tune its lasing frequency, to q-switch or to passively/actively mode lock the laser device in order to obtain pulsed operation.
In general, the resonator-cavity loops (ring-resonators) can be replaced by lo any other implementation of a frequency-selective element that couple between the two waveguide sections. For example, optical gratings can be used.
Thus, according to yet another aspect of the present invention, there is provided an integrated electro-optical device comprising:
a combination of two spaced-apart waveguides and at least two spaced-apart wavelength-selective elements accommodated between the two waveguides and connected to each other through sections of the waveguides, said at least two spaced-apart wavelength-selective elements and said waveguide sections creating a closed loop compound resonator for storing optical energy of a predetermined frequency range; and
a control means for controlling physical characteristics of the compound resonator to adjust its optical storage characteristics.
According to yet another aspect of the present invention there is provided a method for manufacturing the above device utilizing existing lithography techniques.
More specifically, the present invention is used with the ring-resonators and is therefore described below with respect to this application.